It works like this:
Imaginary lines are drawn dividing the image into thirds both horizontally and vertically. You place important elements of your composition where these lines intersect. I've even made a little diagram for you. (below)
Using the Rule of Thirds helps produce nicely balanced movies. Also, as you have to position things relative to the edges of the frame it helps get rid of ' tiny subject surrounded by vast empty space' syndrome.
external image rule-of-the-thirds-slice_09.gif
You do not want to place the person in the middle of the camera. Instead you would want to place the person around the top left hand corner cross. - KSZ